142 The Great World's Farm 



particles, and for precisely the same reason. Both stand 

 in need of iron. Iron oxide is reddish when it contains 

 the full amount of oxygen possible, and green when it 

 contains less. Give the plant iron and keep it in the light, 

 and the grains of leaf-green at once begin to turn their 

 proper color, and tiny grains of starch form within them. 



A very little light, barely enough to read by, will be 

 sufficient to make a plant begin to turn green, but not 

 sufficiently green to enable it to separate the carbon; and 

 therefore in dim light no starch grains can be formed. In 

 ordinary daylight, however, whether the sun be shining 

 directly upon the plant or no, these starch-grains are being 

 continually produced; but the brighter the light the more 

 briskly the manufacture goes on, up to a certain point — 

 provided, that is, that the air contains carbon dioxide 

 wherewith to furnish the necessary supply of carbon. If 

 it does not, no starch, of course, can be formed, no mat- 

 ter how bright the light, or how green the leaf-green. 



But all air, unless artificially deprived of it, contains 

 enough to furnish what the plant requires in this respect, 

 thanks to the supplies which are being constantly furnished 

 to it. This is taken up not only by the leaves, but by all 

 the green parts of a plant, leaves, buds, stems, and fruit, 

 so long as these remain green; for it is only in the cells 

 which contain ieaf-green that starch is manufactured from 

 the gas. These green cells lie immediately underneath the 

 thicker-walled but transparent cells of the surface which 

 compose the skin; and through the skin the gas finds its 

 way into them. 



The leaf-pores, by which water escapes, are openings 

 in the skin formed by two curved, lip-hke cells, which 

 gape open in hot, bright weather, and close more or less 



